We have previously described, for example, in U.S. Pat. Nos. 5,023,064, 4,963,329 and 4,865,817, assigned to the assignee hereof and the disclosures of which are incorporated herein by reference, treatment of gas streams to remove components therefrom, wherein multiple two-phase nozzles located in a horizontal conduit are used to form sprays of fine liquid droplets of absorbing medium contacting the gas stream to absorb a solute gas from the gas stream and a fan is used to coalesce the liquid droplets at the downstream end. In a situation where low levels of particulate material contaminate the gas stream, an agglomerator fan is not necessary and conventional entrainment equipment can be used, as described in copending U.S. patent application Ser. No. 754,643 filed Sep. 4, 1991, assigned to the assignee hereof and the disclosure of which is incorporated herein by reference.
In present practices, the absorption of a solute gas from a gas mixture using a liquid or slurry sorbent is conducted in a continuous flow counter-current fashion. The type of equipment employed is normally a vertical tower containing either packing or trays that enhance the gas-liquid contact surface area. In some instances, for example where liquid slurries form the sorbent phase, open spray towers are used. These spray towers usually generate large droplet sizes, generally at least about 700 .mu.m, to avoid elutriation of the liquid with the gas stream exiting at the top of the column. The solute-containing gas stream is introduced at the bottom of the tower while the scrubbing liquor is introduced at the top of the tower so as to effect counter-current contact. As a consequence of this design, the solute-depleted gas phase is contacted with the lean sorbent at the top of the tower while "loaded" sorbent is in contact with the solute rich gas phase at the bottom of the tower.
Such counter-current contact promotes high levels of removal of the solute gas from the gas stream while producing a "loaded" sorbent stream, assuming that the tower height or the number of theoretical equilibrium stages is adequate. Mass transfer of the solute gas to the liquid sorbent phase is dependent on a number of parameters, the most important of which is the surface area of the liquid exposed to the gas phase. There are many classical approaches to increasing this so-called contact area reported in the literature. For example, the surface area developed by most packed-beds generally averages around 500 ft.sup.2 per US gallon of liquid introduced over the packing via a proper distributor. In recent years, "new" packings have been developed which reportedly increase this area by 20%, but these materials only bring the contact area to approximately 600 ft.sup.2 per gal of liquid.
Two-phase atomizing nozzles as described in U.S. Pat. No. 4,893,752, assigned to the assignee hereof and the disclosure of which is incorporated herein by reference, produce sprays of small droplets that create large numbers of droplets from each gallon sprayed (50 trillion droplets at 5 .mu.m) and consequently very large surface areas. This technology has been applied to in-duct gas-liquid contact where the potential was demonstrated to generate as much as about 50,000 ft.sup.2 per US gallon of liquid sprayed. Aqueous solutions of organic amines that selectively chemisorbed sulfur dioxide (SO.sub.2) from off-gas streams have been sprayed in a series of consecutive stages in counter-current scrubbing and SO.sub.2 removal efficiencies up to 99% have been achieved, as described in the aforementioned U.S. patent application Ser. No. 754,643. In the arrangement described in this prior application, an entrainment separator is located downstream of each nozzle, so that each nozzle, in effect, is located in its own individual compartment, and the amine solution is passed serially to each nozzle, countercurrent to the direction of flow of the gas stream so that a SO.sub.2 -depleted gas stream is exposed to the lean absorbing medium and a partially-loaded absorbing medium is exposed to an increasingly rich gas stream containing SO.sub.2.
Experimental runs showed that the above-described counter-current chemisorption with interstage entrainment separation provided good SO.sub.2 removal while attaining good loading of the absorbent. An apparent drawback of this arrangement is the necessity of spraying the full stream of liquid absorbent sequentially to each of the stages so established which, in effect, results firstly in the consumption of fairly large quantities of compressed air used in the two-phase atomizing nozzles and secondly to the necessity of using one pump for each stage. One novel approach to decrease the consumption of air would be to split the full stream of liquid sorbent into separate streams for each stage, creating parallel flow using only one pump to distribute the appropriate amount of liquor to each nozzle. Consequently, the total process compressed air required for liquid atomization at each nozzle is reduced proportionately to the amount of liquid pumped.